An induction motor/generator is an extremely durable and reliable energy conversion apparatus. Because of these attributes, manufacturers of aircraft electrical power systems have strived for years to make the induction motor/generator a part of an overall hybrid electrical power generating and aircraft engine starting system. A myriad of problems have stood in the way of accomplishing this feat, however. Specifically, since an induction generator requires excitation from an external source in order to function, provision of excitation has been a problem.
U.S. Pat. No. 4,447,737, which issued on May 8, 1984 to Cronin, is directed to a solution for providing excitation for an induction generator used in an aircraft electrical power system. Cronin is directed to a combination induction generator/synchronous generator power system. The power system includes an induction generator and a tandem generator enclosed in a single housing. The tandem generator includes a tandem synchronous generator which optionally excites the induction generator or provides power to the aircraft systems upon activation of a three-phase contactor. The tandem generator also includes a tandem synchronous generator which produces, via a phase controlled rectifier bridge, 270 volt DC power for the aircraft systems.
Apparently, Cronin addresses a two level system whereby a synchronous generator is employed when aircraft electrical loads are relatively minor. When aircraft electrical loads are larger, the synchronous generator is used as an exciter for an induction generator, which then supplies power to the larger loads. If DC output is desired, it is taken directly from a second synchronous generator.
As Cronin reveals, it is desirable to provide aircraft systems which are capable of producing both AC and DC. Cronin uses separate generators to produce AC and DC because an integrated system (one which employs a single generator as a source for both AC and DC power) has traditionally been subject to distortion due to AC and DC interaction.
In a similar vein is an apparatus described in U.S. Pat. No. 4,684,873, which issued on Aug. 4, 1987 to Glennon, directed to a hybrid generating system comprising an AC power generating section driven by a prime mover for generating AC output power and a DC power generating section independent of the AC power generating section, also driven by the prime mover for generating the DC output power. Each of the AC and DC power generating sections includes a permanent magnet generator. Glennon teaches full separation of AC and DC power sources, because, in the past, DC power provided by integrated systems has had less than desirable reliability. AC power produced by such integrated systems has been distorted because of rectification of a part of the AC power to produce the DC power. Such integrated hybrid systems have suffered from reduced efficiency.
In a related case, U.S Pat. No. 3,267,353, which issued on Aug. 16, 1966 to Franklin, is directed to a duplex generator system and more particularly to such a system for providing independently regulated alternating current and direct current output. Franklin notes that it has not been found practical in the past to utilize a single regulated alternating current source and rectify and regulate a portion of the power to provide a direct current source, since rectification of a part of the power results in substantial distortion in the alternating current wave shape. Franklin asserts that two completely separate generating systems would provide the independently regulated sources without wave shape distortion, but obviously such separate systems would be more expensive than a single combined magnetic structure.
Along similar lines, U.S. Pat. No. 4,330,743, which issued on May 18, 1982 to Glennon, and is commonly assigned with the subject invention, is directed to an electrical aircraft engine start and generating system for use in an aircraft having an engine driven torque converter coupled to an alternator which provides AC power for conversion to DC and AC power. This system includes a reversible AC to DC converter controllably electrically coupled to the alternator and a controller unit to provide DC power in a generating mode. The reversible AC to DC converter is capable of receiving externally supplied DC power to be converted to AC power to drive the alternator as a motor in start mode. A DC to AC converter is controllably electrically coupled to the controller unit and to the DC power output during the generating mode. The reversible DC to AC converter in the start mode is mutually controllably electrically coupled to the externally supplied DC power. The controller unit and the alternator cooperate to provide a controlled AC power output to be delivered to the alternator to bring the alternator operating as a motor up to operating speed, whereupon the reversible DC to AC converter responds to the external DC power and is electrically coupled to the alternator to drive the alternator as a motor to deliver rotary power through the torque converter to start the aircraft engine.
The subject invention differs from that disclosed in Glennon in two material respects. First, Glennon employs a constant speed drive between a variable speed aircraft engine and the alternator/motor. The constant speed drive drives the alternator/motor at a constant speed. Because the alternator/motor is driven in a constant speed, a high frequency AC link between the alternator/motor and the hybrid AC/DC converter is not needed. Second, Glennon is directed to use of a synchronous alternator/motor, wherein a more rugged and dependable induction motor/generator is used in the subject invention. In essence, the subject invention represents an evolutionary step over the invention found in Glennon.
Recent improvements in computer controlled solid state power conversion systems have allowed advances to occur in power conversion which now make an induction generator an appropriate source of power for an integrated hybrid AC/DC aircraft electrical power system. An integrated system employing an induction generator as its power source would enjoy benefits from vastly increased simplicity, reliability and ruggedness of the generator and would allow multiple forms of electrical energy to be derived from the system without requiring duplication of elements therein.
Accordingly, subject invention is the first to employ an induction generator as the heart of an integrated hybrid AC/DC electrical power generating system. Further, the subject invention employs the induction generator as a motor to allow starting of an aircraft engine by either an external AC and DC power source.